There is a requirement in industry for the measurement of temperature at all points over long distances. Typical uses are for monitoring long cables and pipelines. As these structures may be very long, there is a need for a sensing system that operates over very long distances. There is a requirement in communications and sensing to measure the loss along optical fibres. As these fibres may be very long, there is a need for a loss measurement system that operates over very long distances.
Distributed temperature sensors usually use Raman scattering in optical fibres as the means to determine the temperature. Here, light from a laser source is sent down a fibre and the small amount of light that is scattered back towards the source is analysed. By using pulsed light and measuring the returning signal as a function of time, the backscattered light that was generated at all points along the fibre can be determined. This backscattered light contains components that are up- and down-shifted in frequency from the source light (Raman and Brillouin anti-Stokes and Stokes light respectively) and light that is elastically scattered (Rayleigh light). The powers of the returning Raman and Brillouin signals are temperature dependent and so analysis of these components yields the temperature. Usually, the Raman Stokes and anti-Stokes signals are used to determine the temperature however sometimes the Rayleigh light is used as a reference and sometimes the Brillouin components are used. The Rayleigh light, and sometimes the Raman Stokes light, is often used to measure the loss along an optical fibre.
An example is known from U.S. Pat. No. 5,194,847 relating to fibre optic intrusion sensing. In this case, for sensing intrusion into a predefined perimeter, a coherent pulsed light is injected into an optical sensing fibre positioned along the predefined perimeter. A backscattered light in response to receiving the coherent light pulses is produced and coupled into an optical receiving fibre. The backscattered light is detected by a photodetector and an intrusion is detectable by a change in the backscattered light. To increase the sensitivity of the apparatus, a reference fibre and an interferometer may also be employed.
As the fibre length increases, the resolutions of the temperature and loss measurements become poorer. This is because there are causes of loss in an optical fibre that attenuate the signal. As the length of the fibre is increased, the overall loss over the length of the fibre increases and so the signal returning from the far end is smaller and, as a consequence, noisier. An obvious solution to this problem is to launch more light into the fibre to compensate for the losses but there is a limit to how much light may be launched. This is because when high power light is sent down a fibre, there are non-linear effects that become significant as the length of the fibre is increased. The most problematic of these is the stimulated Raman effect. This takes power from the source light and shifts it to the wavelength of the Raman Stokes signal. It is usually this stimulated Raman scattering (SRS) that limits the length over which a distributed sensor of this type may operate. It is known from “Optical Time Domain Reflectometry”, Hartog, Arthur, Harold, and WO 1998 GB 0000028, Jan. 8, 1997, to attempt to alleviate this problem by proposing fibres that make the SRS threshold occur at higher input powers so that more power may be usefully used in the fibre. This approach is restrictive, however, as it requires the technique be used in potentially expensive, specialised fibres. This approach will also not allow the system to be used on normal optical fibres that may have previously been installed.
Another cause of poorer resolution of the temperature and loss measurements as the fibre length increases is the longer round trip delay for each pulse. It is normally only possible to usefully have one pulse in the fibre at any time as otherwise it would not be possible to determine where the returning signal was generated (the backscattered signals from the multiple pulses would overlap). The time for a light pulse to travel along a fibre is proportional to the length of the fibre and so, as the length of the fibre is increased, the time between the pulses that can be launched has to be increased. As typically many averages are required to measure the signal with a reasonable accuracy, this necessary reduction in pulse repetition rate means that the measurements become less accurate as the sensing length is increased. However, the SRS threshold cannot be raised indefinitely and the pulse repetition rate is still limited by the round-trip time of the pulse through the whole length of fibre.
Another source of error is differential loss at different wavelengths. This is a property of most common types of fibre and is a source of error because the temperature calculation involves determining a ratio of powers of the returning light at different wavelengths. The Raman Stokes and anti Stokes components are shifted to different wavelengths and so suffer different amounts of loss. As the distance of the desired measurement point along the fibre increases, this source of error becomes more significant. It is sometimes possible to reduce the error by choosing a launch wavelength which minimises the differential loss for the given fibre. It is also possible to use two optical sources or a single tunable source to transmit at two different wavelengths, take measurements using the stokes wavelength from one and the anti stokes wavelength for the other, to cancel out the differential loss. This is shown in U.S. Pat. No. 4,767,219. This can cancel the differential loss error, but it would be difficult or expensive to ensure the two sources are sufficiently similar over their lifetime that other errors are not introduced. The single tunable source is less practical because it is difficult to tune and pulse such sources.